To take advantage of the information in the mass spectra of a compound, it is necessary to create ions. Conventional mass spectrometers operate on a beam of ions derived from the material to be analyzed, by deflecting the beam electromagnetically, electrostatically or by in a way which depends upon the mass-to-charge ratio of ions in the beam or by measuring the transit times of ions in a pulsed beam. Specimens to be analyzed with a mass spectrometer are generally converted to ions before introduction into the portion of a mass spectrometer that performs the mass analysis of the ion beam.
Ionizing liquid samples has been an ongoing challenge, especially as the demand for applicability and sensitivity has been ever increasing. Many samples of interest are not sufficiently volatile at ambient temperatures to form a gas, nor are they sufficiently thermally stable at elevated temperatures to change state from a liquid to a gas without thermal decomposition. The state change is usually required with most conventional liquid inlet systems. More than 90 percent of the 12 million compounds in the American Chemical Society Registry of Organic Compounds are unable to change state without thermal decomposition. This limitation precludes the application of ionization methods such as bombardment of gas molecules with very high energy ions or electrons to form additional ions such as in electron impact ionization, or introducing chemically reactive high energy ions into the gas phase such as Fast Atom Bombardment (FAB) or Desorption Chemical Ionization. In research applications such as field toxin analysis, or in applications requiring ultra sensitivity (such as the detection of small molecules of glycine in amino acid analysis), the analysis poses ever increasing challenges.
Techniques exist for generating ion vapors from compounds of low volatility (i.e. electrospray ionization, field ionization, laser assisted field desorption, plasma desorption). Creating ions through external application of a charge is not uniformly successful, as not all compounds can be ionized to a high degree. To date, all known methods have limitations and none is useful for all types of compounds.